Dermatological composition

ABSTRACT

The invention relates to dermatological compositions comprising a multiplicity of particles, at least one of the particles comprising porous and/or polycrystalline silicon. The dermatological compositions covered by the invention include compositions for sun protection applications. The use of porous silicon, polycrystalline silicon, and porous silicon oxide mirrors is disclosed.

This invention relates to a new dermatological composition. Morespecifically this invention relates to a new dermatological compositioncomprising silicon and/or silicon oxide.

[0001] The term “dermatological composition” covers a huge range ofproducts that may be applied to the skin. Such products may be used forthe treatment of skin conditions or damaged skin; they may be used toprotect the skin, or for cosmetic purposes; they may even be used totreat organs located beneath the skin.

[0002] Dermatological compositions are applied to skin in a number ofdifferent forms including creams, pastes, powders, and gels, and mayinclude components such as oils, pigments, fillers, surfactants,emollients, basifying/acidifying agents, fragrances, pharmaceuticalproducts, and particulates.

[0003] Particulates may be used in dermatological compositions for avariety of reasons. They may be present as a filler, which increases thevolume of the skin composition. Fillers commonly used in skin creamsinclude talc, mica, and silica. Alternatively the particulate may confercolour to the cream, say for cosmetic purposes. The particulate may be abeneficial substance that protects or has a beneficial effect upon theskin, or that has a beneficial effect on organs lying below the skin.Finally the particulate may be a carrier that interacts with abeneficial substance in such a manner that the effect of the beneficialsubstance is enhanced as a result of the interaction. For example acarrier may assist in transfer of a beneficial substance through or intothe skin, or it may protect a beneficial substance from other componentsin the composition.

[0004] As is well known exposure to direct sunlight may cause sun-burn,ageing effects, and skin cancer. Cosmetic sunscreen preparations aim toreduce this risk and often include chemical compounds which are capableof absorbing certain wavelengths of UV radiation. Many also incorporatea particulate component having diameters of 10-150 nm whose purpose isto reflect or scatter certain wavelengths of UV radiation whileremaining optically transparent on the skin. Although effective, UVabsorbing chemicals may cause sensitisation in some users. The smallparticles used are typically of a metal oxide, with titanium and zincoxides being particularly common.

[0005] A problem associated with the use of titanium dioxide is that ithas a relatively high refractive index, which causes whitening whenapplied to the skin. To overcome this problem ultrafine particles oftitanium dioxide have been used, which cause less scattering in thevisible region. Unfortunately such ultrafine particles (of titaniumoxide, or zinc oxide) suffer from problems of coagulation which cancause deterioration of their optical properties. A further problemassociated with titanium and zinc oxides is their hydrophilicity,reducing the water repellent properties of any composition of which theyform a part.

[0006] Dermatological compositions often have to satisfy a number ofcriteria such as cosmetic appearance, water resistance, and stability.The composition should be comfortable to apply and continue to feelcomfortable once applied, for example it should not cause an allergicreaction and should not cause sensitisation of the skin.

[0007] The property of stability (the retention of the physical andchemical properties of the composition after manufacture) includesstability prior to and after application to the skin. A common cause ofcomposition instability is the agglomeration of one or more of thecomponents. Agglomeration of a solid component, comprising for examplenanoparticles, may result in an abrasive or unpleasant sensation whenthe composition is applied to the skin.

[0008] There are thus many problems, or potential problems, to be takeninto account when selecting products for topical application to humanskin; and it is an objective of this invention to provide a newdermatological composition that ameliorates at least some of theseproblems.

[0009] According to a first aspect, the present invention provides adermatological composition comprising a multiplicity of dermatologicalparticles, at least one of the dermatological particles comprisingporous and/or polycrystalline silicon.

[0010] Preferably the dermatological composition comprises amultiplicity of silicon particles each silicon particle comprising oneor more of: bulk crystalline silicon, porous silicon, amorphous silicon,and polycrystalline silicon.

[0011] Porous and/or polycrystalline silicon, when located in amammalian body, may be degraded to yield silicic acid, H₄SiO₄. Clinicalevidence suggests that exposure to silicic acid is of value in thetreatment of skin complaints such as psoriasis and conditions associatedwith the biological ageing of the skin, hair and nails. For example,studies by Lassus, in the Journal of International Medical Research,1997, vol. 25, pp. 206-209 and 1993, vol. 21, pp. 209-215, discuss theresults of tests involving the oral and topical administration ofcolloidal silicic acid to groups of patients. Significant improvementswere observed in skin thickness and turgor; alleviation of the symptomsof chronic plaque-type psoriasis was also observed.

[0012] The effects of supplementing the diets of calves with silicicacid, H₄SiO₄, has been reported by Colomme et al., Biological TraceElement Research, 1997 vol. 56, p153. A 4.9% increase in dietary siliconover a 23 week period led to a 70% increase in serum siliconconcentration and produced a significant increase in dermal collagenlevels.

[0013] The present invention, therefore provides a composition which issuited for use as a pharmaceutical product to deliver silicic acid viathe skin. The compostion is suitable for use as a treatment forconditions affecting the skin, such as psoriasis and also those whichaffect the underlying bones. and joints, such as osteoporosis.

[0014] Thin films of porous silicon show good optical transparency, forexample a 20 μm thick film with porosity of 90% provides greater than95% transmission in the visible region. Furthermore, the absorption ofUV radiation by porous silicon is high, for example the absorptioncoefficient above the direct bandgap of Si (3.25 eV or 400 nm) is around5×10⁴ cm⁻¹ for a material with a porosity of 90%. This far exceeds theUV absorption ability of commonly used metal oxide nanoparticles such asTiQ₂ which has an absorption coefficient of ca. 10³ cm⁻¹ at 400 nm.Porous silicon is therefore suitable for use as a UV radiation screeningagent in a sunscreen preparation.

[0015] The dermatological composition may further comprise adermatologically acceptable carrier. Preferably the dermatologicalcarrier comprises a natural or synthetic fat; the fat may be an oil or awax.

[0016] High porosity porous silicon has a refractive index comparable tomany commonly used dermatological carriers, making its concealment bythe carrier more effective.

[0017] The dermatological composition may comprise one or more of: afatty binder, a filler, a pigment, a volatile oil, an anti-oxidant, asurfactant, a basifying agent, an acidifying agent, and a fragrance.

[0018] The dermatological composition may have a composition such thatit is one of: a cream, a lotion, a topical solution, a paste, alinament, a powder, a gel, a tincture, and an aerosol.

[0019] For pharmaceutical use small quantities of porous and/orpolycrystalline silicon may be effective. Porous and/or polycrystallinesilicon may comprise less than 2% by weight with respect to the totalweight of the composition. Preferably, the porous and/or polycrystallinesilicon comprises at least 2% by weight with respect to the total weightof the composition.

[0020] Suitably the or each porous and/or polycrystalline siliconparticle has a particle size in the range from 0.01 to 250 μm,preferably, the particle size is in the range from 0.01 to 0.15 μm andmore preferably the particle size is in the range from 0.01 to 0.03 μm.

[0021] Advantageously the dermatological composition comprises amultiplicity of silicon particles and each silicon particle has aparticle size in the range from 0.01 to 250 μm. More advantageously,each silicon particle has a particle size in the range from 0.01 to 0.15μm. Yet more advantageously each silicon particle has a particle size isin the range from 0.01 to 0.03 μm.

[0022] The particle size range chosen is dependent on the intended useof the composition. For use in a sunscreen applications, it may bedesirable for the particles to have a size that scatters ultravioletwavelengths. The upper bound to the particle size of the porous siliconis limited by the need for the composition to be comfortable in use.Particles which are too large will result, in the composition beinggranular to the touch and which may cause abrasion of the skin onapplication. The dermatological composition may be used as a deodorant.The dermatological composition may also be used as a coloured lipstick.Finally, cosmetic aspects may also have to be taken into account, theparticle size influencing the appearance of the composition on the skin.

[0023] The extent of porosity of the porous silicon has been shown bythe inventors to affect the rate at which it is absorbed by themammalian body. Highly porous silicon is more rapidly resorbed thanlower porosity silicon. The porosity is measured in terms of thefractional void content of the porous silicon.

[0024] Preferably, the dermatological composition comprises poroussilicon and the porous silicon has a porosity of at least 1%, morepreferably, the porous silicon has a porosity of at least 50%.

[0025] Porous silicon may be classified according to the nature of theporosity. Dependent on the conditions of manufacture, it may bemicroporous, with an average pore size of less than 20 Å; mesoporous,with an average pore size of between 20 and 500 Å; or macroporous withan average pore size of greater than 500 Å.

[0026] For use in the present invention, preferably the dermatologicalcomposition comprises porous silicon and the porous silicon ismicroporous or mesoporous, more preferably it is mesoporous.

[0027] Preferably the porous and/or polycrystalline silicon isresorbable.

[0028] Advantageously the dermatological composition further comprisesat least one beneficial substance.

[0029] For the purposes of this specification a beneficial substance isdefined as a substance that has a beneficial effect when administered tothe body of an animal or human. The beneficial substance may be locatedwithin the structure of the porous and/or polycrystalline silicon. Ifthe beneficial substance is resorbable, then corrosion of the porousand/or polycrystalline silicon may result in release of the beneficialsubstance.

[0030] The beneficial substance may be one or more of: a pharmaceuticalmaterial, a biological material, a genetic material, a radioactivematerial, an antibacterial agent or a luminescent material.

[0031] The beneficial substance may be one or more of: insulin,lidocaine, alprostadil, calcitonin, DNA, RNA, tumour necrosis factor(TNS), a peptide, cytokine, a hormone, an antibody, a cytotoxic agent,an adjuvant, a steroid, an antibiotic, a cinamate derivative, octylmethoxycinnamate, a Salicylate, a benzophenone, an anthranilate, adibenzoylmethane, a p-aminobenzoate, S a vitamin C derivative, a βcarotene, an α tocopherol, a thiol, an antifungal agent, an antiviralagent, and a psoralen.

[0032] Another beneficial substance may be a protein, for examplecollagen. It is reputed that collagen has anti-ageing attributes whenapplied to the skin. A further beneficial substance may be a vitamin,for example vitamin E. A yet further beneficial substance may be a tracemineral. A non-exhaustive list of suitable trace minerals includes;selenium, manganese, molybdenum, chromium, vanadium, iodine, fluorineand cobalt.

[0033] In general, trace minerals are needed by the body only in verysmall quantities, the recommended daily allowance (RDA) of certain traceelements can be less than 0.1 mg/day. The effects of deficiencies arehowever, well documented. Furthermore, some trace minerals are requiredin specific areas of the body. Delivery of trace minerals in the form ofa topical application is therefore attractive.

[0034] A still further beneficial substance may be a therapeuticelement. Possible therapeutic elements include, lithium, gold, silver,copper, zinc, and platinum. These elements have widespread clinicalusage for the treatment of chronic conditions; lithium is used to treatdepression, silver and gold have antibacterial properties, copper andzinc are commonly applied via the skin, and platinum is used in thetreatment of neoplastic diseases. It may also be desirable to be able todeliver these elements to specific sites via topical application.

[0035] It is of course intended that the porous silicon may have morethan one beneficial substance incorporated within its structure.

[0036] Preferably the dermatological composition comprises derivatisedporous silicon. For the purposes of this specification derivatisedporous silicon is defined as porous silicon having a monomolecular, ormonatomic layer that is chemically bonded to at least part of thesurface, including the surface of the pores, of the porous silicon. Thechemical bonding, between the layer and the silicon, may comprise a Si—Cand/or Si—O—C bonding.

[0037] In this way the beneficial substance may be bonded to the surfaceof the porous silicon. The porous silicon may also be derivatised so asto enhance wetting of the porous and/or polycrystalline siliconparticles by other components contained in the dermatologicalcomposition.

[0038] Advantageously the dermatological composition comprises siliconoxide, more advantageously the dermatological composition comprisesporous and/or polycrystalline silicon oxide. Yet more advantageously thedermatological composition comprises porous silicon part of which hasbeen oxidised to form silicon oxide.

[0039] The dermatological composition may comprise a multiplicity ofmirrors, each mirror comprising a plurality of layers, each layercomprising porous and/or polycrystalline silicon.

[0040] By having a plurality of porous layers, each layer having adifferent porosity to layer(s) adjacent to it, and by controlling theporosity of each layer, the mirror may be made to reflect radiation overa limited range of wavelengths. The dermatological composition maycomprise a first group of mirrors that reflect radiation over a firstwavelength range, and a second group of mirrors that reflect radiationover a second wavelength range. The dermatological composition maycomprise several groups of mirrors, each group reflecting radiation overa wavelength range that differs from that of the other groups.

[0041] The use of mirrors that reflect different wavelengths may havetwo advantages. Firstly it potentially affords protection over a greaterrange of wavelengths. Secondly it opens the way for coloration of thecomposition using the reflective properties of the mirrors; in this waythe composition may be used to darken the skin causing it to appeartanned.

[0042] Preferably each mirror may have a largest dimension in the range50 nm to 2 mm. More preferably each mirror has a largest dimensionbetween 100 nm and 1 mm.

[0043] Each mirror may have a largest dimension that is greater thantwice its smallest dimension.

[0044] Advantageously each mirror comprises resorbable porous silicon.

[0045] The mirrors may reflect electromagnetic radiation away from theskin of a human to which the dermatological composition has beenapplied. If each mirror is resorbable then they will gradually corrodeupon application to the skin. This corrosion will be linked to a changein the appearance of the composition, and this change can be used as anindicator of the composition effectiveness.

[0046] Preferably the or each dermatological particle comprises siliconoxide.

[0047] According to a second aspect the invention provides adermatological composition comprising a multiplicity of silicon oxidemirrors, each silicon oxide mirror comprising a plurality of layers,each layer comprising porous and/or polycrystalline silicon oxide.

[0048] By having a plurality of porous layers, each layer having adifferent porosity to layer(s) adjacent to it, and by controlling theporosity of each layer, the mirror may be made to reflect radiation overa limited range of wavelengths. The dermatological composition maycomprise a first group of mirrors that reflect radiation over firstwavelength range, and a second group of mirrors that reflect radiationover a second wavelength range. The dermatological composition maycomprise several groups of mirrors, each group reflecting radiation overa wavelength range that differs from that of the other groups.

[0049] Preferably each silicon oxide mirror may have a largest dimensionin the range 50 nm to 2 mm. More preferably each silicon oxide mirrorhas a dimension between 100 nm and 1 mm.

[0050] Each mirror may have a largest dimension that is greater thantwice its smallest dimension.

[0051] The dermatological composition may further comprise adermatologically acceptable carrier. Preferably the dermatologicalcarrier comprises a natural or synthetic fat; the fat may be an oil or awax.

[0052] Porous silicon oxide has a refractive index comparable to that ofcommonly used dermatologiocal carriers, making its concealment by thecarrier more effective.

[0053] Preferably the porous silicon oxide is resorbable.

[0054] The dermatological composition may comprise one or more of: afatty binder, a filler, a pigment, a volatile oil, an anti-oxidant, asurfactant, and a fragrance.

[0055] The dermatological composition may have a composition such thatit is one of a cream, a lotion, a topical solution, a paste, a linament,a powder, a gel, a tincture or an aerosol.

[0056] According to a third aspect the invention provides a method oftherapeutic or prophylactic treatment of a human or animal bodycomprising the steps of: topically applying a dermatological compositionand allowing the dermatological composition to treat the human oranimal; the dermatological composition comprising a multiplicity ofdermatological particles, at least one of the dermatological particlescomprising porous and/or polycrystalline silicon.

[0057] According to a fourth aspect the invention provides a method ofcosmetic treatment of a human or animal body comprising the steps of:topically applying a dermatological composition and allowing thedermatological composition to cosmetically treat the human or animal;the dermatological composition comprising a multiplicity ofdermatological particles, at least one of the dermatological particlescomprising porous and/or polycrystalline silicon.

[0058] According to a fifth aspect the invention provides a method oftherapeutic or prophylactic treatment of a human or animal bodycomprising the steps of: topically applying a dermatological compositionand allowing the dermatological composition to treat the human oranimal; the dermatological composition comprising a multiplicity ofsilicon mirrors, at least one of the mirrors comprising a plurality ofporous and/or polycrystalline silicon oxide layers, each layer having adifferent refractive index to the layer or layers adjacent to it.

[0059] According to a sixth aspect the invention provides a method ofcosmetic treatment of a human or animal body comprising the steps of:topically applying a dermatological composition and allowing thedermatological composition to cosmetically treat the human or animal;the dermatological composition comprising a multiplicity of mirrors, atleast one of the mirrors comprising a plurality of layers of porousand/or polycrystalline silicon oxide, each layer having a refractiveindex that differs from that of the layer or layers adjacent to it.

[0060] According to a seventh aspect the invention provides a method ofmanufacturing a dermatological composition comprising the step ofcombining at least one particle, comprising porous and/orpolycrystalline silicon, with a dermatologically acceptable carrier.

[0061] According to a eighth aspect the invention provides a method ofmanufacturing a dermatological composition comprising the step ofcombining at least one mirror, the mirror comprising a plurality oflayers of porous and/or polycrystalline silicon oxide each layer havinga refractive index that differs from that of the layer or layersadjacent to it, with a dermatologically acceptable carrier.

[0062] According to a ninth aspect, the invention provides a method ofprotecting at least part of an animal or human from electromagneticradiation, comprising the steps:

[0063] (a) applying a dermatological composition to at least part of theskin of the animal or human; and

[0064] (b) allowing, when at least part of the skin is exposed toelectromagnetic radiation, the dermatological composition to reflect atleast part of the electromagnetic radiation;

[0065] wherein step (b) comprises the step of reflecting at least partof the electromagnetic radiation by means of a multiplicitydermatological mirrors.

[0066] Each dermatological mirror may comprise a plurality of poroussilicon and/or porous silicon oxide layers. Each porous silicon and/orporous silicon oxide layer may have a different porosity to that of itsneighbouring layer or layers. Each dermatological mirror may comprise aplurality of porous silicon and/or porous silicon oxide layers havingalternating high and low porosities. The low porosity layers may eachhave a porosity between 25% and 65%, and the high porosity layers mayeach have a porosity between 60% and 95%. Each dermatological mirror maycomprise greater than 10 layers. Each dermatological mirror may comprisegreater than 100 layers. Each dermatological mirror may comprise greaterthan 200 layers. Each dermatological mirror may comprise greater than orequal to 400 layers.

[0067] Each layer of porous silicon and/or porous silicon oxide, fromwhich the mirrors may be formed, has a different refractive index to itsneighbouring layer or layers, the combined layers forming a Bragg stackmirror.

[0068] The dermatological composition may comprise a dermatologicallyacceptable carrier such as a natural or synthetic fat. The multiplicityof dermatological mirrors may be in the form of a powder, the powderbeing distributed throughout the carrier.

[0069] Preferably each dermatological mirror comprises one or more of:bulk crystalline silicon, porous silicon, amorphous silicon, andpolycrystalline silicon.

[0070] Advantageously each dermatological mirror comprises siliconoxide.

[0071] The dermatological composition may be a sun cream, providingprotection against ultraviolet radiation.

[0072] Each dermatological mirror may comprise porous silicon, and atleast part of the porous silicon may comprise porous polycrystallineand/or amorphous silicon.

[0073] Step (b) may comprise the step of allowing the multiplicity ofdermatological mirrors to reflect, with a reflectivity peak between 5 nmand 380 nm, at least part of the electromagnetic radiation.

[0074] Step (b) may comprise the step of allowing the multiplicity ofdermatological mirrors to reflect, with a reflectivity peak between 380nm and 780 nm, at least part of the electromagnetic radiation.

[0075] Step (b) may comprise the step of allowing the multiplicity ofdermatological mirrors to reflect, with a reflectivity peak between 740nm and 100 μm, at least part of the electromagnetic radiation.

[0076] Step (b) may comprise the step of allowing the multiplicity ofdermatological mirrors to reflect, with a reflectivity peak between 400nm and 800 nm, at least part of the electromagnetic radiation.

[0077] Step (b) may comprise the step of allowing the multiplicity ofdermatological mirrors to reflect, with a reflectivity peak between 550nm and 700 nm, at least part of the electromagnetic radiation.

[0078] Preferably the average particle size of the multiplicity ofdermatological mirrors is in the range 50 nm to 2 mm. More preferablythe average size of the multiplicity of the dermatological mirrors isbetween 100 nm and 1 mm.

[0079] The average particle size of the multiplicity of dermatologicalmirrors may be in the range 10 nm to 50 microns. The average particlesize of the multiplicity of dermatological mirrors may be in the range200 microns to 1 mm.

[0080] The reflectance properties of the multiplicity of dermatologicalmirrors may impart colour to the dermatological composition, or they canbe used to reflect undesirable radiation from the skin to which thecomposition is applied.

[0081] According to a tenth aspect, the invention provides adermatological composition comprising a plurality of dermatologicalmirrors.

[0082] Each dermatological mirror may comprise a plurality of poroussilicon and/or porous silicon oxide layers. Each porous silicon and/orsilicon oxide layer may have a different porosity to that of itsneighbouring layer or layers. Each dermatological mirror may comprise aplurality of porous silicon and/or porous silicon oxide layers havingalternating high and low porosities. The low porosity layers may eachhave a porosity between 25% and 65%, and the high porosity layers mayeach have a porosity between 60% and 95%. Each dermatological mirror maycomprise greater than 10 layers. Each dermatological mirror may comprisegreater than 100 layers. Each dermatological mirror may comprise greaterthan 200 layers. Each dermatological mirror may comprise greater than orequal to 400 layers.

[0083] The dermatological composition may be a sun-cream.

[0084] Preferably each dermatological mirror comprises one or more of:bulk crystalline silicon, porous silicon, amorphous silicon, andpolycrystalline silicon.

[0085] Each dermatological mirror may comprise silicon oxide. Eachdermatological mirror may comprise porous silicon oxide.

[0086] Each dermatological mirror may comprise porous silicon, and theporous silicon may comprise porous polycrystalline and/or porousamorphous silicon.

[0087] Preferably the dermatological composition comprises adermatologically acceptable carrier. Advantageously the dermatologicalcarrier may comprise a natural or synthetic fat such as an oil or wax.The plurality of dermatological mirrors may be distributed substantiallyuniformly throughout the volume of the dermatological carrier.

[0088] The orientation of each dermatological mirror may besubstantially random.

[0089] Each dermatological mirror may comprise porous silicon and/orporous silicon oxide, the porous silicon and/or porous silicon oxidehaving a structure such that, when each dermatological mirror issubstantially randomly oriented, the plurality of dermatological mirrorshas a peak in its reflectivity spectrum between100 nm and 380 nm.

[0090] Each dermatological mirror may comprise porous silicon and/orporous silicon oxide, the porous silicon and/or porous silicon oxidehaving a structure such that, when each dermatological mirror issubstantially randomly oriented, the plurality of dermatological mirrorshas a peak in its reflectivity spectrum between 380 nm and 780 nm.

[0091] Each dermatological mirror may comprise porous silicon and/orporous silicon oxide, the porous silicon and/or porous silicon oxidehaving a structure such that, when each dermatological mirror issubstantially randomly oriented, the plurality of dermatological mirrorshas a peak in its reflectivity spectrum between 740 nm and 100 μm.

[0092] Each dermatological mirror may comprise porous silicon and/orporous silicon oxide, the porous silicon and/or porous silicon oxidehaving a structure such that, when each dermatological mirror issubstantially randomly oriented, the plurality of dermatological mirrorshas a peak in its reflectivity spectrum between 400 nm and 800 nm.

[0093] Each dermatological mirror may comprise porous silicon and/orporous silicon oxide, the porous silicon and/or porous silicon oxidehaving a structure such that, when each dermatological mirror issubstantially randomly oriented, the plurality of dermatological mirrorshas a peak in its reflectivity spectrum between 550 nm and 700 nm.

[0094] Each dermatological mirror may comprise porous silicon and/orporous silicon oxide, the porous silicon and/or porous silicon oxidehaving a structure such that, when each dermatological mirror issubstantially randomly oriented, the plurality of dermatological mirrorshas a peak in its reflectivity spectrum between 630 nm and 700 nm.

[0095] Each dermatological mirror may substantially consist of poroussilicon.

[0096] Each dermatological mirror may substantially consist of poroussilicon oxide.

[0097] Preferably the average particle size of the plurality ofdermatological mirrors is in the range 50 nm to 2 mm. More preferablythe average particle size of the plurality of the dermatological mirrorsis between 100 nm and 1 mm.

[0098] The average size of the average particle size of the plurality ofdermatological mirrors may be in the range 10 nm to 50 microns. Theaverage size of the average particle size of the plurality ofdermatological mirrors may be in the range 200 microns to 100 μm.

[0099] At least some of the dermatological mirrors may comprise siliconoxide having a structure such that it is soluble in human and/or animalsweat. At least some of the dermatological mirrors may comprise siliconoxide having a structure such that it is soluble in simulated humanand/or animal sweat.

[0100] At least some of the dermatological mirrors may comprise siliconhaving a structure such that it is soluble in human and/or animal sweat.At least some of the dermatological mirrors may comprise silicon havinga structure such that it is soluble in simulated human and/or animalsweat.

[0101] At least some of the dermatological mirrors may comprise abeneficial substance.

[0102] The corrosion of silicon and/or silicon oxide, which may formpart of the dermatological composition, in sweat has several advantages.As mentioned above, dermatological mirrors may impart colour to thedermatological composition. Corrosion of the mirrors in sweat couldtherefore result in a colour change in the composition, a colour changethat could be used to monitor the efficacy of the composition, andindicate the need for further application of the composition.

[0103] Advantageously the dermatological composition comprises avolatile material. The volatile material may comprise more than onecompound. The volatile material may be a liquid or solid at 20 C and 760mm Hg. Preferably the volatile material is a liquid at 20 C and 760 mmHg. The volatile material may have a volatility such that, when 9 g+/−1g is disposed within a layer of mesoporous silicon, the mass lossthrough evapouration at 20 C and 760 mm Hg is greater than or equal to0.01 mg per minute over the first two minutes of measurement.

[0104] The volatile material may be lavender oil and/or Tea Tree Oil.

[0105] The dermatological mirrors may be of value in imparting a colourto the dermatological composition; they may even give the composition aglittering or glinting appearance. At least part of the volatilematerial may be distributed in or on the silicon and/or silicon oxide,from which the dermatological composition is at least partly formed.

[0106] Advantageously the silicon, from which the plurality ofdermatological mirrors are at least partly formed, comprises poroussilicon, and at least some of the volatile material is disposed in atleast some of the pores of the porous silicon.

[0107] According to an eleventh aspect, the invention provides a methodof delivering a beneficial substance to or through at least part of theskin of an animal or human, comprising the steps of:

[0108] (a) applying a dermatological composition, comprising abeneficial substance, to at least part of the skin of the animal orhuman; and

[0109] (b) allowing the beneficial substance to be released to orthrough the skin of the animal and/or human;

[0110] wherein the dermatologal composition comprise silicon and/orsilicon oxide, at least part of the beneficial substance being locatedin or on at least part of the silicon and/or silicon oxide, and whereinstep (b) comprises the step of allowing the silicon and/or silicon oxideto corrode in sweat excreted from the skin of the animal or human,thereby releasing the beneficial substance.

[0111] The dermatological composition may comprise silicon and thesilicon may be selected from one or more of bulk crystalline silicon,porous silicon, amorphous silicon, and polycrystalline silicon.Preferably the dermatological composition comprises porous silicon.

[0112] The dermatological composition may comprise silicon oxide, andthe silicon oxide may be porous silicon oxide.

[0113] The dermatological composition may comprise porous silicon andthe porous silicon may be porous polycrystalline and/or amorphoussilicon.

[0114] The dissolution of silicon and/or silicon oxide in sweat meansthat the beneficial substance associated with it may be released as aresult of the dissolution.

[0115] The step of allowing the porous silicon and/pr silicon oxide tocorrode may comprise the step of allowing the porous silicon and/orsilicon oxide to corrode less than or equal to 2 hours after contactwith the sweat.

[0116] The step of allowing the porous silicon and/or porous siliconoxide to corrode may comprise the step of allowing the porous siliconand/or silicon oxide to corrode less than or equal to 6 hours aftercontact with the sweat.

[0117] The beneficial substance may comprise silicon or a siliconcompound. The beneficial substance may be formed, at least partly fromat least some of the porous silicon.

[0118] The beneficial substance may be one or more of: a pharmaceuticalmaterial, a biological material, a genetic material, a radioactivematerial, an antibacterial agent or a luminescent material.

[0119] The beneficial substance may be one or more of: insulin,lidocaine, alprostadil, calcitonin, DNA, RNA, tumour necrosis factor(TNS), a peptide, cytokine, a hormone, an antibody, a cytotoxic agent,an adjuvant, a steroid, an antibiotic, a cinamate derivative, octylmethoxycinnamate, a Salicylate, a benzophenone, an anthranilate, adibenzoylmethane, a p-aminobenzoate, a vitamin C derivative, a βcarotene, an α tocopherol, a thiol, an antifungal agent, an antiviralagent, and a psoralen.

[0120] Another beneficial substance may be a protein, for examplecollagen. It is reputed that collagen has anti-ageing attributes whenapplied to the skin. A further beneficial substance may be a vitamin,for example vitamin E. A yet further beneficial substance may be a tracemineral. A non-exhaustive list of suitable trace minerals includes;selenium, manganese, molybdenum, chromium, vanadium, iodine, fluorineand cobalt.

[0121] A still further beneficial substance may be a therapeuticelement. Possible therapeutic elements include, lithium, gold, silver,copper, zinc, and platinum.

[0122] The silicon, from which the dermatological composition may be atleast partly formed, may comprise a multiplicity of silicon particles.At least some of the silicon particles may comprise porous silicon.

[0123] The silicon oxide, from which the dermatological composition mayat least partly be formed, may comprise a multiplicity or silicon oxideparticles.

[0124] According to a twelfth aspect, the invention provides adermatological composition comprising a multiplicity of porous siliconparticles, each porous silicon particle comprising an outer layer;characterised in that the outer layer has a composition such that itcorrodes, when it is placed in contact with the skin of an animal orhuman.

[0125] The outer layer may comprise gelatin.

[0126] According to an thirteenth aspect the invention provides adermatological composition comprising a multiplicity of dermatologicalparticles.

[0127] Preferably each dermatological particle comprises one or more of:bulk crystalline silicon, porous silicon, amorphous silicon, andpolycrystalline silicon. More preferably each dermatological particlecomprises porous silicon.

[0128] Certain forms of porous silicon, are photoluminescent, emittingvisible light when illuminated with ultraviolet radiation. It maytherefore be used as a marker to determine whether a beneficialsubstance has been administered by the application of a dermatologicalcomposition. Particles of small particles of porous silicon, invisibleunder normal conditions, may photoluminesce when the skin of an animalor human is exposed to the UV radiation.

[0129] Advantageously each dermatological particle comprises siliconoxide. More advantageously each silicon oxide particle comprises poroussilicon oxide.

[0130] The dermatological composition may further comprise adermatologically acceptable carrier. Preferably the dermatologicalcarrier comprises a natural or synthetic fat; the fat may be an oil or awax. The multiplicity of silicon and/or silicon oxide particles may bedistributed throughout the carrier.

[0131] According to a fourteenth aspect, the invention provides the useof silicon and/or silicon oxide for the manufacture of a medicament forthe delivery of a beneficial substance to or through at least part ofthe skin of an animal or human.

[0132] The step of delivering a beneficial substance to or through atleast part of the skin, may comprise the steps of applying adermatological composition, comprising the beneficial substance, to atleast part of the skin of the animal or human and allowing thebeneficial substance to be released to or through the skin of the animaland/or human.

[0133] The dermatological composition may comprise at least part of thesilicon and/or silicon oxide.

[0134] At least part of the beneficial substance may be located in or onat least part of the silicon and/or silicon oxide and the step ofallowing the beneficial substance to be released may comprise the stepof allowing the silicon and/or silicon oxide to corrode in sweatexcreted from the skin of the animal or human, thereby releasing thebeneficial substance.

[0135] Silicon, used to deliver the beneficial substance, may beselected from one or more of bulk crystalline silicon, porous silicon,amorphous silicon, and polycrystalline silicon. Preferably the siliconcomprises porous silicon.

[0136] Silicon oxide, used to deliver the beneficial substance, may beporous silicon oxide.

[0137] The dissolution of silicon and/or silicon oxide in sweat meansthat the beneficial substance associated with it may be released as aresult of the dissolution.

[0138] The step of allowing the porous silicon and/or silicon oxide tocorrode may comprise the step of allowing the porous silicon and/orsilicon oxide to corrode less than or equal to 2 hours after contactwith the sweat.

[0139] The step of allowing the porous silicon and/or porous siliconoxide to corrode may comprise the step of allowing the porous siliconand/or silicon oxide to corrode less than or equal-to 6 hours aftercontact with the sweat.

[0140] The beneficial substance may comprise silicon or a siliconcompound. The beneficial substance may be formed, at least partly fromat least some of the porous silicon.

[0141] The beneficial substance may be one or more of: a pharmaceuticalmaterial, a biological material, a genetic material, a radioactivematerial, an antibacterial agent or a luminescent material.

[0142] The beneficial substance may be one or more of: insulin,lidocaine, alprostadil, calcitonin, DNA, RNA, tumour necrosis factor(TNS), a peptide, cytokine, a hormone, an antibody, a cytotoxic agent,an adjuvant, a steroid, an antibiotic, a cinamate derivative, octylmethoxycinnamate, a Salicylate, a benzophenone, an anthranilate, adibenzoylmethane, a p-aminobenzoate, a vitamin C derivative, a βcarotene, an α tocopherol, a thiol, an antifungal agent, an antiviralagent, and a psoralen.

[0143] Another beneficial substance may be a protein, for examplecollagen. It is reputed that collagen has anti-ageing attributes whenapplied to the skin. A further beneficial substance may be a vitamin,for example vitamin E. A yet further beneficial substance may be a tracemineral. A non-exhaustive list of suitable trace minerals includes;selenium, manganese, molybdenum, chromium, vanadium, iodine, fluorineand cobalt.

[0144] A still further beneficial substance may be a therapeuticelement. Possible therapeutic elements include, lithium, gold, silver,copper, zinc, and platinum.

[0145] The silicon, from which the dermatological composition may be atleast partly formed, may comprise a multiplicity of silicon particles.At least some of the silicon particles may comprise porous silicon.

[0146] The silicon oxide, from which the dermatological composition mayat least partly be formed, may comprise a multiplicity or silicon oxideparticles.

[0147] The invention will now be described by way of example only withreference to the following drawings, in which:

[0148]FIG. 1 shows a dermatological composition, according to theinvention, comprising a multiplicity of particles in contact with humanskin;

[0149]FIG. 2 shows one of the particles illustrated in FIG. 1;

[0150]FIG. 3 shows transmission spectra and absorption spectra comparingbulk crystalline silicon with porous silicon with differing degrees ofporosity;

[0151]FIG. 4 shows an SEM image of a porous silicon mirror prior toimmersion in simulated human sweat;

[0152]FIG. 5 shows an SEM images of a porous silicon mirror afterimmersion in simulated human sweat for periods of two, six, and twentyhours;

[0153]FIG. 6 shows two SEM images, of porous silicon mirrors afterimmersion in simulated human sweat for the same period of time, theimmersion being under different conditions for each mirror;

[0154]FIG. 7 shows reflectance spectra for porous silicon, which is inthe form of a film, and also in the form of a powder;

[0155]FIG. 8 shows the effect of oxidation on the reflectance spectrumof porous silicon mirror;

[0156]FIG. 9a shows a SEM image of a gelatine coated porous siliconmirror impregnated with Tea Tree Oil;

[0157]FIG. 9b shows EDX plots for a gelatine coated porous siliconmirror;

[0158]FIG. 10a shows an SEM image of a polycrystalline layer depositedon a silica substrate; and

[0159]FIGS. 10b and 10 c show SEM images of porosified polycrystallinelayers.

[0160] Preparation of a First Dermatological Composition

[0161] A first dermatological composition, according to the invention,may be prepared by the following process:

[0162] Step A1

[0163] Porous silicon was fabricated by anodising a heavily boron dopedCZ silicon wafer with an initial resistivity of 0.01-0.03 Ωcm. Theanodisation was carried out in an electrochemical cell, as described inU.S. Pat. No. 5,348,618 containing an electrolyte comprising a 10%solution of hydrofluoric acid in ethanol. An anodisation current withdensity of 50 mA cm⁻² was passed for 12 minutes. This produced a 20 μmthick porous silicon layer with a porosity of 90% and a mass density of0.47 mg cm⁻².

[0164] Step B1

[0165] The porous silicon layer may then be detached from the underlyingwafer by increasing the current density, for a few seconds, to asufficiently high value that the silicon at the interface between theporous and bulk crystalline regions is completely dissolved. It was thensupercritically dried as described by Canham in Nature, vol. 368, (1994)p 133-135.

[0166] Step C1

[0167] To produce porous silicon in a particulate form suitable for usein a dermatological composition the detached layer was subjected toultrasonic agitation in a liquid known to allow good dispersion ofsilicon powders. Common organic liquids such as alcohols, ketones oraldehydes were found to be suitable. It was then necessary to reduce thesize of the silicon particles. This was achieved using a centrifuge asdescribed by Mizuta et al. in Ceramic Bulletin, vol. 61, (1982), p872-875. The porous silicon particulate layer was suspended inn-propanol and centrifuged at 12000 rpm to remove large agglomerates.The supernatant was then centrifuged at 17000 rpm to give a dispersionof nanoparticulate porous silicon.

[0168] Alternatively, the particle size may be reduced by milling, asdescribed by Kerkar et al. in Jn. Am Ceram. Soc., vol 73, (1990), p2879-2885. The detached layer is mixed with an organic liquid such astrichloroethylene and subjected to attrition milling in a Si₃N₄container using Si₃N₄ balls for several hours.

[0169] Step D1

[0170] After removal of the organic liquid and drying, the powderedporous silicon was suitable for use in a dermatological composition. Itwas used either without further processing or with one or morebeneficial substances.

[0171] There are a number of methods by which a beneficial substance maybe associated with the silicon particles to be used in thedermatological composition. The beneficial substance may be dissolved orsuspended in a suitable solvent, the silicon particles may then beincubated in the resulting solution for a period of time.

[0172] The beneficial substance may be deposited on the surface of theimplants. If the implants comprise porous silicon, then a solution ofthe beneficial substance may be introduced into the pores of the poroussilicon by capillary action. Similarly if the silicon particles have acavity then the solution may also be introduced into the cavity bycapillary action. If the beneficial substance is a solid but has asufficiently high vapour pressure at 20 C then it may be sublimed ontothe surface of the silicon particles. If a solution or suspension of thebeneficial substance can be formed then the substance may be applied tothe silicon particles by successive immersion in the solution/suspensionfollowed by freeze drying.

[0173] The particle size of the porous silicon was measured bytransmission electron microscopy however any other suitable method suchas laser Doppler particle size analysis, light scattering or Stokessettling techniques may be used.

[0174] Step E1

[0175] A dermatological composition was prepared by blending powderedporous silicon with a dermatologically acceptable carrier, such that thepercentage of porous silicon comprised about 2% by weight of thecomposition. If the carrier comprises an oil, then the silicon particlesmay be sprinkled into the oil with stirring to homogenise the mixture.The oil/porous silicon mixture may then be combined with othercomponents, to form the dermatological composition.

[0176] Preparation of a Second Dermatological Composition

[0177] A second dermatological composition, according to the invention,may be prepared by the following process:

[0178] Step A2

[0179] A 0.2 Ωcm p⁻ CZ Si substrate is anodized in H₂O:HF:C₂H₅OHelectrolyte, the components of the electrolyte being present in theratios 1:1:2 by volume. Current density is periodically modulated from30 mAcm⁻² to 120 mAcm⁻² to yield low (corresponding to 30 mAcm⁻²) andhigh (corresponding to 120 mAcm⁻²) porosity layers. Anodisation in thisway results in the formation of a silicon multilayer mirror. By varyingthe time intervals over which the two current densities flow, the stopband (the spectral region of high reflectivity) of the mirror can betuned across the visible range and into the ultraviolet.

[0180] Typical approximate reflectivities of the as-etched poroussilicon mirrors are 99% at 740 nm, 90% at 490 nm, and 50% at 370 nm.

[0181] Step B2

[0182] A multiplicity of particulate silicon mirrors may then beobtained from the single multilayer mirror film by repeating steps B1 toE1 recited for the first dermatological composition. In this way themultilayer mirror is detached from the silicon substrate, formed intosilicon particles (at least some of the particles being a mirror) eitherultrasonically or by crushing, combined with one or more beneficialsubstances, and finally combined with a suitable dermatological carrier.

[0183]FIG. 1 shows a number of particulate mirrors 11 that are incontact with the skin 12. Each particulate mirror 11 comprises aplurality of layers of porous silicon (see FIG. 2). Radiation 13, sayUV-A radiation, is reflected by the mirrors 11 before the radiation 13reaches the skin 12. The particulate mirrors 11 form part of adermatological composition, which also comprises a dermatologicallyacceptable carrier 14.

[0184]FIG. 2 shows one of the particulate mirrors shown in FIG. 1,generally indicated by 13. The particulate mirror 13 comprises a firstgroup of layers 21 and a second group of layers 22, the porosity of thefirst group 21 differing from that of the second group 22.

[0185] Preparation of a Third Dermatological Composition

[0186] A third dermatological composition, according to the invention,may be prepared in the following manner:

[0187] Step A3

[0188] The step A2, given for preparation of the second dermatologicalcomposition, is repeated with modulation between 80 mAcm⁻² to 120mAcm⁻². The porous silicon multi-layer mirror may then be detached fromthe underlying wafer by increasing the current density, for a fewseconds, to a sufficiently high value that the silicon at the interfacebetween the porous and bulk crystalline regions is completely dissolved.The mirror is then supercritically dried as described by Canham inNature, vol. 368, (1994) p 133-135 and thermally oxidised by heating themultilayer mirror in dry oxygen at 950 C. for five minutes to yield aporous silicon oxide multilayer mirror. (M Berger et al, Thin SolidFilms, Vol 297, p237-240, 1997).

[0189] Step B3

[0190] A multiplicity of porous silicon oxide mirrors may then beobtained from the single silicon oxide multilayer mirror by repeatingsteps C1 to E1 recited for the first dermatological composition. In thisway the silicon oxide multilayer mirror is detached from the substrate,formed into particles either ultrasonically or by crushing, combinedwith one or more beneficial substances, and and finally combined with asuitable dermatological carrier.

[0191] The hydrophilicity of the porous silicon oxide mirror may beincreased by wet oxidation of the porous silicon at step A3. Byincreasing the hydrophilicity in this way, aqueous solutions may be usedfor the ultrasonic and centrifuging processes associated with step C1.

[0192] Preparation of a Fourth Dermatological Composition

[0193] A fourth dermatological composition, according to the invention,may be prepared in the following manner:

[0194] Step A4

[0195] A (100) p-type boron doped wafer with resistivity of 0.01 Ωcm isanodised galvanostatically at 37 mAcm⁻² in a 1:1 by volume mixture of48% HF:C₂H₅OH for 3 hours in the dark to yield a single 300 μm thicklayer of porous silicon. This single layer of porous silicon has anaverage porosity of 65%. A high current etch of 400 mAcm⁻² for 5 minutesthen releases the porous layer as a free standing film.

[0196] Subsequent rinsing with ethanol and excess dry hexane was thencarried out without permitting intermediate drying of the wafers.Derivatization was then carried out, using a Lewis acid (EtAlCl₂)mediated hydrosilylation to replace the silicon hydride termination ofthe wafers. Hydrosilylation was carried out with 1 dodecyne and yieldeda dodecenyl terminated surface. The Lewis acid mediated hydrosilylationwas performed in the following manner:

[0197] A hexane solution of the Lewis acid (EtAlCl₂) is bought intocontact with the surface of the freshly anodized sample of poroussilicon (comprising a single layer of uniform porosity). 1 dodecyne isthen also placed on the surface of the porous silicon and the consequentreaction is allowed to proceed at an ambient temperature of 20 C. for aperiod of at least 2 hours. The sample is then quenched with THF,followed by CH₂Cl₂. The whole process, from the application of the Lewisacid through to the quenching with CH₂Cl₂ is performed in an inertatmosphere. The derivatized sample is then rinsed in ethanol and driedunder an N₂ stream. The resulting surface is capped with a monolayer ofdodecenyl groups.

[0198] Step B4

[0199] A multiplicity of silicon particles, each of which comprisesderivatised porous silicon, may then be obtained from the single layerof derivatised porous silicon by repeating steps C1 to E1 recited forthe first dermatological composition. In this way the single layer ofderivatised porous silicon is detached from the substrate, formed intoparticles either ultrasonically or by crushing, combined with one ormore beneficial substances, and finally combined with a suitabledermatological carrier.

[0200] Preparation of a Fifth Dermatological Composition

[0201] A fifth dermatological composition, according to the invention,may be prepared by the following process:

[0202] A layer of polycrystalline silicon was deposited on a glasssubstrate by pyrolysis of SiH₄ at 0.3 torr, at 600 to 620 C., in aThermco TMX9000 low pressure chemical vapour deposition hot walledfurnace. The glass substrate is then removed by etching in aqueous HFsolution to obtain a free standing layer. The polycrystalline layer maythen be formed into a multiplicity of silicon particles by ultrasonictreatment or milling as described step C1 and combined with adermatologically acceptable carrier as described at step E1.

[0203] Alternatiely, the multiplicity of polycrystalline siliconparticles may be porosified by stain etching prior to combination withthe dermatologically acceptable carrier.

[0204] Preparation of a Sixth Dermatological Composition

[0205] A sixth dermatological composition, according to the invention,may be prepared by the following process:

[0206] A mirror comprising multiple layers of polycrystalline siliconmay be deposited on a substrate by PECVD of hydrogen diluted SiH₄ usingelectron cyclotron resonance at temperatures less than 200 C. (seeKalkan et al J. Appl Phys, Vol 88, p 555-561 (2000)). The microwavepower is modulated periodically with time to generate a number of layersof polycrystalline silicon, each layer having a refractive index thatdiffers from those of its adjacent layer(s). The process is performed ata pressure between 5 and 12 mtorr, a silane flow rate of 2 sccm, and ahydrogen flow rate of 40 sccm.

[0207] The polycrystalline mirror may then be formed into a multiplicityof particulate mirrors by ultrasonic treatment or milling as describedstep C1 and combined with a dermatolocially acceptable carrier asdescribed at step E1.

[0208] Transmission Verses Wavelength Characteristics of Porous Silicon

[0209]FIG. 3 shows transmission T and absorption coefficient A versesphoton energy E spectra for porous and bulk crystalline silicon. Plot 31corresponds to a 90% porous silicon layer, which effectively screens UVradiation with photon energies of 3.87-3.25 eV (320-400 nm) whileallowing transmission of lower energy, longer wavelength visible light.Plot 32 corresponds to 75% porosity porous silicon; a comparison ofplots 31 and 32 shows how the transmission properties may be altered byaltering the porosity of the porous silicon. Improved UV screening canbe achieved by sacrificing some optical transparency and vice versa. Theoptical characteristics of crystalline bulk silicon, shown by plot 33,displays UV screening capabilities, but shows very poor opticaltransparency.

[0210] Stability of Porous Silicon in Sweat

[0211] 10 repeat multilayer silicon mirrors, each containing 10 lowporosity and 10 high porosity porous silicon layers, were fabricated byanodisation at modulated current density for a total of 156 seconds in20% ethanoic HF.

[0212] Each of the 10 repeat mirrors was immersed in simulated humansweat (SHS) for varying periods of time. The preparation of simulatedhuman sweat was in accordance with ISO standard (3160/2) and isdescribed by J P Randin in J. Biomed. Mater. Res. 22, 649 (1988). Thesimulated sweat comprises NaCl (20 g/litre), NH₄Cl (17.5 g/litre), urea(5 g/litre), acetic acid (2.5 g/litre) and lactic acid (15 g/litre). ThepH was adjusted to 5.5 by addition of NaOH.

[0213]FIG. 4 shows an SEM image of one of the 10 repeat silicon mirrorsprior to immersion in the simulated human sweat.

[0214]FIG. 5 shows SEM images of one of the ten repeat mirrors afterimmersion in SHS for a period of 2, 6, and 20 hours at 25 C. After 2hours (FIG. 5(a)) there is minimal change in thickness, but the topmosthigh porosity layer has undergone significant attack, after 6 hours(FIG. 5(b)) there are 18 of the 20 layers remaining, whilst after 20hours (FIG. 5(c)) half of the structure has completely dissolved and theremainder has been affected throughout its depth to become partiallydelaminated.

[0215]FIG. 6 shows two mirrors, one of which (FIG. 6(a)) has beenimmersed in SHS at 25 C. for a period of 2 hours, and one of which (FIG.6(b)) has been immersed in SHS at 37 C. for 2 hours. The mirror that hadbeen immersed at 37 C. was also simultaneously illuminated with; UVradiation (2.5 mWcm⁻², 365 nm). The illumination with UV radiation atelevated temperatures simulates sunbathing conditions. Comparison ofFIGS. 6(a) and 6(b) shows that illumination with UV and increase intemperature causes an increase in the corrosion rate.

[0216] Reflectance Properties of Porous Silicon and Oxidised PorousSilicon

[0217] An ultrathick free standing mirror of 300 repeats (600 layers)was fabricated by anodisation in 20% ethanoic HF, at a modulated currentdensity for a total of 1 hour and 8 minutes (0.7 amps for 9 secondintervals and 4.55 amps for 4.5 second intervals, the switch between lowand high current not being instantaneous).

[0218] Viewed at normal incidence, the front face had a red hue, therear face a vivid green colour. FIG. 7(a) shows the reflectivityspectrum taken from the front face of the intact 150 micron thick filmusing Ocean Optics S2000 system. The peak in reflectivity at about 650nm is consistent with its red appearance. The film was then crushed intoa powder with a pestal and mortar. The average particle size wasapproximately 500 microns. FIG. 7(b) shows a reflectivity spectrum fromthe powder, under conditions where a number of randomly orientedparticles contributed to the signal.

[0219] A thinner mirror of the same microstructure, fabricated by thesame method as that used for the 300 repeat mirror, but having only 100repeats (200 layers), and still adhered to its silicon substrate, wassubjected to partial oxidation.

[0220]FIG. 8a shows the reflectance spectrum for the 100 repeat mirror,attached to its substrate. After oxidation the mirror became pale purplein colour and had a blue-shifted reflectance peak shown in FIG. 8b.Further oxidation would shift the reflectance peak out of the visibleregion and into the near UV region.

[0221] Volatile Agents: Loading of and Release from Porous Silicon

[0222] (A) Lavender Oil

[0223] A large flake of a porous silicon membrane, with a dry weight of8.0+/−0.1 mg, was immersed at 20 C in 1 ml of Meadows pure concentratedlavender oil (Lavendula Officinalis) for a period of 10 minutes. Theflake was then given a brief water rinse. Residual oil on the externalsurfaces of the flake was then removed by contact with filter paper, andthe flake was re-weighed. In its initial as-impregnated state the weightof the flake plus lavender oil was 17.3+/−0.2 mg, falling by 1 mg in thefirst 2 minutes but much more gradually thereafter. After 2 hours in airthe weight had fallen to 8.6+/−0.1 mg. The measurements of mass losswere conducted at 20 C and 760 mm Hg.

[0224] (B) Tea Tree Oil

[0225] A segment of a 100 repeat multilayer mirror attached to its bulksilicon substrate was impregnated with Tea Tree Oil (MelaleicaAlternifolia). After pre-warming the layer to 60 C. on a hot plate, adrop of concentrated oil was pippetted onto the surface. There was animmediate colour change from green to red and a gradual increase in thediameter of the circular red region of the mirror as the oil dropletspread laterally across the outer surface, in addition to infiltratingthe film. Extended application of a stream of dry nitrogen gas to theaffected region caused the original green colour to gradually reappear.

[0226] The initial application of the oil was repeated on anothersegment, but this time a very thin gelatine film was deposited over theimpregnated region to entrap the volatile oil within. This was achievedby warming 2 g of gelatine in 200 ml of water to 45 C. and then dippingthe segment into this solution and allowing the adsorbed film tosolidify by cooling to room temperature in air. The coated segment wasfound to retain its red colour. FIG. 9a shows an SEM image of thegelatine coated mirror. FIG. 9b shows EDX plots for the top (FIG.9b(i)), middle (FIG. 9b(ii)), and bottom (FIG. 9b(iii)) of the layer ofporous silicon containing the Tea Tree Oil and capped with gelatine. TheEDX plots show that the oil (indicated by the carbon and oxygen peaks)has penetrated throughout the layer of porous silicon.

[0227] Alternative Methods of Impregnation

[0228] A sample of porous silicon, either attached to or detached from abulk crystalline silicon substrate, may be disposed in a sealedcontainer above a volatile liquid. The vapour pressure within the poroussilicon may slowly be increased by heating the volatile material betweenits melting point and its boiling point. This appraoch may be of valuefor volatile liquids that do not wet porous silicon or which are proneto solidification on the pore walls.

[0229] Preparation of Porous Polycrystalline Silicon

[0230]FIG. 10a shows a SEM image of a polycrystalline film of silicon,deposited onto silica by thermal decomposition of silane at 620 C. in aLPCVD reactor. Segments, were subjected to a stain etch in a 50:1mixture of 40 wt % HF and 70% nitric acid respectively. After 15seconds, about half the layer was porosified (as shown in the SEM imageof FIG. 10(b); after 30 seconds the electrolyte had penetrated throughthe full thickness of the layer and has started to create voids in theunderlying oxide (FIG. 10(c)).

[0231] The use of porous polycrystalline silicon in dermatologicalcompositions is of value, since it is much less expensive to fabricatethan porous silicon derived from bulk crystalline silicon.

1. A dermatological composition comprising a multiplicity of particles,at least one of the particles comprising porous and/or polycrystallinesilicon.
 2. A dermatological composition according to claim 1 whereinthe composition further comprises a dermatologically acceptable carrier.3. A dermatological composition according to claim 1 wherein theporous/and or polycrystalline silicon is resorbable.
 4. A dermatologicalcomposition according to claim 1 wherein the composition comprisesderivatised porous silicon.
 5. A dermatological composition according toclaim 1 wherein the composition comprises porous silicon oxide.
 6. Adermatological composition according to claim 1 wherein at least part ofthe porous silicon has been oxidised to form silicon oxide.
 7. Adermatological composition according to claim 1 wherein the compositioncomprises a multiplicity of mirrors, each mirror comprising a pluralityof layers, each layer comprising porous silicon.
 8. A dermatologicalcomposition according to claim 7 wherein the composition comprises aplurality of groups of mirrors, each group reflecting radiation over awavelength range that differs from that of the other groups.
 9. Adermatological composition according to claim 1 wherein the compositionfurther comprises at least one beneficial substance.
 10. Adermatological composition comprising a multiplicity of silicon oxidemirrors, each oxide mirror comprising a plurality of layers, each layercomprising porous silicon oxide.
 11. A dermatological compositionaccording to claim 10 wherein composition comprises a plurality ofgroups of mirrors, each group reflecting radiation over a wavelengthrange that differs from that of the other groups.
 12. A dermatologicalcomposition according to claim 10 wherein at least one of said mirrorscomprises resorbable porous silicon oxide.
 13. A dermatologicalcomposition according to claim 10 wherein the composition furthercomprises at least one beneficial substance.